I. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a gallium phosphide semiconductor device having improved electrodes.
II. Description of the Prior Art
Various semiconductor devices based on compound semiconductive materials are known. Among them are light emitting diodes, semiconductor lasers and microwave elements such as Gunn diodes. The most popular device of these is a light emitting diode the substrate of which is made of gallium phosphide (GaP) monocrystal. This light emitting diode is used as a display element for watches and a display element for meters for automobile and industrial use.
A monocrystal semiconductor alone cannot perform any useful operation. It does not work as a semiconductor device unless it is provided with electrodes as well as a semiconductor layer and supplied with drive energy through the electrodes. A GaP light emitting diode, for example, comprises an N type GaP monocrystal substrate, an N type layer epitaxially grown on the substrate, a P type layer also epitaxially grown on the N type layer, and a pair of electrodes formed on the P type layer and on the exposed surface of the substrate, respectively. Generally, a voltage is applied between the electrodes thereby to make the PN junction emit light.
Material of the electrodes of such a GaP light emitting diode must satisfy various requirements. First, it must form a good ohmic contact with a compound semiconductor crystal (e.g. GaP crystal). Second, it can be well bonded to lead wires. Third, it can be formed in a fine pattern. Further, it must be heatresistive and durable at 500.degree. C. or more. Still further, it must be strong against chemicals, particularly acids. Moreover, it can be shaped in the form of a film and prepared in a large quantity.
Generally, electrodes formed on an N type GaP substrate are made of a 2%Si-98% Au alloy film. This Si-Au alloy satisfies almost all the above-mentioned requirements. But it cannot be prepared in a sufficiently large quantity. In addition, an interface between this Si-Au alloy and the substrate becomes black at a heat treatment. Therefore, if provided with electrodes made of such a Si-Au alloy, a light emitting semiconductor device would inevitably have its light emitting efficiency reduced.